In an AM receiver or an FM receiver, an AGC (automatic gain control) circuit is used for the sake of preventing variations in an output voice level according to receiving field intensity. For instance, the AGC circuit is connected to an intermediate-frequency amplifier circuit included in the AM receiver, and the gain of the intermediate-frequency amplifier circuit is controlled according to an AM detection output. Thus, the gain of the intermediate-frequency amplifier circuit is set at a large value in a weak field area, and is inversely set at a small value in a strong field area so as to constantly obtain an almost fixed voice output.
The above-mentioned AGC circuit in the past requires a DC level to be detected by smoothing sound signals, and so a low-pass filter of a large time constant is used. To be more specific, it is necessary to set an element constant of a capacitor or a resistance constituting the low-pass filter at a large value, and there is a problem that, considering an increase in area occupied by these elements, the entire AGC circuit cannot be integrally fabricated with other circuits on a semiconductor substrate.